Re: psql - -dry-run option

On 18 December 2015 at 16:50, Shulgin, Oleksandr <
oleksandr(dot)shulgin(at)zalando(dot)de> wrote:

> I'm on the same line: BEGIN/ROLLBACK requires trivial effort and a
> --dry-run option might give a false sense of security, but it cannot
> possibly rollback side-effects of user functions which modify filesystem or
> interact with the outside world in some other way.
>

Pretty much. Side effects.

The most glaringly obvious example is nextval(...) and setval(...). You
can't make them have no effect, since your script will then fail to run.
But you can't roll them back either.

Also, anything that touches the file system, like COPY. Untrusted PLs that
can fiddle with the file system. FDWs. All sorts. Oh, and of course psql
commands like \o .

I think this idea is completely unworkable. You might work around the
filesystem access issues with a new attribute (like LEAKPROOF) that asserts
that a function relies strictly on in-database transactional behaviour. But
you're not going to be able to do a dry run with sequence setting. You
could ignore setval and run nextval as normal, but then your dry-run
wouldn't reflect what the real run would do...

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services